The invention relates generally to radiation shielding or attenuating systems and more particularly to a method and system which enables workers to work in an enclosed radioactive workspace safely for long periods with freedom of movement in the workspace.
In nuclear power plants, steam generators typically are utilized as heat exchangers between the reactor and a power generating steam turbine. The steam generators have hundreds or thousands of heat exchange tubes in single pass or double pass loop tubes inside the generator housing, which typically are sixty feet high. The tubes carry contaminated water, at high temperature and pressure from the reactor, through the generators, which in turn transfers heat to water around the tubes, creating steam to drive the turbine. Periodically, typically during a reactor refueling outage, or if a leak occurs, the tubes are checked to make sure they are not leaking or stressing to the point where they will leak contaminated water into the steam.
The tubes are checked by running a tester, typically an eddy current tester, over the length of each tube to be checked. Typically, unless a leak has occurred, some predetermined number of the thousands of tubes are checked during each outage so that during a period of time all the tubes are checked. The testers are operated and repairs or sealing off of leaking tubes are sometimes performed by workers physically climbing into the steam generators through a manway or portal into the bottom of the generator housing. The bottom of the generator housing or the workspace is typically called the channel head and generally is a quarter of a sphere (a half of a bowl) with a diameter of nine to twelve feet. The interior of the channel head presents workers with a main problem of a high radiation environment and a secondary problem of contact contamination with radioactive airborne particles which are on the interior surfaces of the housing and channel head and are dislodged by the workers themselves from the tubes and surfaces.
Due to the fact that the interior or workspace of the generator channel head is a high radiation environment, the workers are only permitted to be inside the generator for a few minutes at a time. The checking and repairs, therefore may require hundreds of entrances and exits. Many attempts to shield the radiation of the interior walls and tubes of the generator have been attempted with limited success. Most of these shielding approaches are very expensive and create radioactive wastes.
Further, the problem of contact contamination within the workspace requires respiratory protection for the workers, as well as causing surface contamination of the workers garments, which contamination is then brought out of the generator when the workers climb out of the portal. Each worker typically is attired in several layers of clothing covered by a plastic outer layer and a self contained breathing apparatus or supplied-air respirator. This makes it difficult to pass through the portals, which usually are small on the order of sixteen inches in diameter, and the clothing itself is cumbersome to work in.
Further, each time the worker leaves the generator at least the outer clothing which has radioactive contaminants on it has to be removed and disposed of. The area around the portal becomes contaminated and must be cleaned. The workers outside the generator are exposed to the contaminants brought out on the clothing and the worker himself is further exposed because the contaminants are in physical contact with the clothing and remain there while the clothing is further handled and disposed of.
The invention permits the workers to work in a radiation attenuated environment and with a decrease in the physical contact with the surfaces of the workspace once the system is assembled. The workers can work in the enclosed workspace of the steam generator channel head inside the system of the invention with a decrease in the physical exposure to the contaminants and a decrease in the carrying of contaminants outside the workspace. The radiation emitted by the surfaces and tubes of the generator, particularly the lower end portion of the tubes, is absorbed by a radiation attenuating medium of the system except where exposed for work on the generator. Thus, the shielding greatly reduces the radiation exposure per unit of time and also can reduce the contamination of workers and outside areas.